Thermal barrier coatings, components, method and apparatus for determining past-service conditions and remaining life thereof

ABSTRACT

A method for determining past-service conditions and/or remaining useful life of a component of a combustion engine and/or a thermal barrier coating (“TBC”) of the component comprises providing a photoluminescent (“PL”) material in the TBC, directing an exciting radiation at the TBC, measuring the intensity of a characteristic peak in the emission spectrum of the PL material, and correlating the intensity of the characteristic peak or another quantity derived therefrom to an amount of a new phase that has been formed as a result of the exposure of the component to extreme temperatures. An apparatus for carrying out the method comprises a radiation source that provides the exciting radiation to the TBC, a radiation detector for detecting radiation emitted by the PL material, and means for relating a characteristic of the emission spectrum of the PL material to the amount of the new phase in the TBC, thereby inferring the past-service conditions or the remaining useful life of the component.

[0001] This invention was first conceived or reduced to practice in theperformance of work under contract DE-FC26-01NT41021 awarded by theUnited States Department of Energy. The United States of America mayhave certain rights to this invention.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to thermal barrier coatings, amethod, and an apparatus for determination of past-service conditions ofcoatings and parts and remaining life thereof. In particular, thepresent invention relates to such a method and an apparatus by anon-destructive optical determination of a particular crystalline phasein a thermal barrier coating.

[0003] The constant demand for increased operating temperature in gasturbine engines has necessitated the development of ceramic coatingmaterials that can insulate the turbine components such as turbineblades and vanes from the heat contained in the gas discharged from thecombustion chamber for extending the life of such components. Theseceramic coatings are known in the art as thermal barrier coatings.

[0004] A thermal barrier coating typically comprises at least a layer ofa refractory or thermally insulating material such as yttria-stabilizedzirconia (or “YSZ”) which is zirconia stabilized with, for example,about 6-8 percent by weight of yttria. The refractory material wouldgenerally be selected to have a low thermal conductivity such as about1-3 W/(m)(K), thereby reducing heat transfer to and the temperatureexperienced by the turbine engine component. The coating may be appliedby one of known deposition techniques such as the thermal or plasmaspray process or the physical vapor deposition process. A typicalthermal barrier coating is a multilayer system comprising three layers.A first so-called bondcoat is applied to the surface of the superalloyof the turbine component. This bondcoat typically comprises a MCrAlYalloy wherein M is nickel, or cobalt, or PtNiAl alloys. The purpose ofthe bondcoat is to provide a layer which adheres well to the underlyingalloy, which provides protection against oxidation of the alloy, andwhich provides a good base for further coatings. A second intermediatelayer or interlayer is applied on the bondcoat. A suitable material forthis interlayer is Al₂O₃. This material can be formed by oxidizing thesurface of the bondcoat to form an oxide layer. The interlayer providesimproved adhesion for the final thermal insulating YSZ coating and isnot included for a thermal barrier property.

[0005] Despite great care taken during manufacture to ensure goodadhesion of the thermal barrier coating to the underlying material ofthe turbine component, thermal cycling during use of such a componenteventually leads to spalling of the coating. In addition, erosion of thethermal barrier coating is inevitable over an extended period of use.Such a spalling or erosion would eventually expose the underlying alloyto extreme temperatures that would lead to failure of the component.Therefore, thermal barrier coatings need be inspected frequently for anysign of deterioration. Such an inspection often requires taking theengine component out of service and is time-consuming. A commoninspection technique is the visual inspection of the presence or absenceof coating. While that method determines when a spall has occurred, itis unable to determine either the degree of deterioration in an intactcoating. A method for determining the past-service conditions andremaining life of thermal barrier coatings would be welcome in the art.

[0006] Similarly, it is desirable to monitor the condition of theturbine components themselves. In the prior art, it is usual for adestructive evaluation to be performed at each inspection interval forcritical components in the hot gas path. In that case, one part isdestroyed to produce sections for metallographical examination. Thecondition of the coatings and base materials are determined frommetallographical inspection, and a decision to repair or replace theremaining parts is made from that information.

[0007] Better knowledge of the past-service conditions experienced bythe turbine components would allow the determination of the remaininglife of a part without destructive evaluation. Currently there are fewin-situ measurements of hot gas path parts temperatures available. Somephysical changes in the phases and structures of the materials ofthermal barrier coatings and components occur with exposure to hightemperatures. Inspection for changes in phase content is one way todetermine past-service conditions.

[0008] However, traditional methods of inspection, such as X-raydiffraction and neutron diffraction, require destructive testing andspecialized equipment. They are not conducive to being deployed at thesite of a gas turbine. In addition, such destructive testing methodsnecessarily extrapolate the result obtained for one part to thecondition of other similarly used parts and, thus, may not provide atrue and accurate condition of those parts.

[0009] European patent application EP 0863396 A2 discloses anon-destructive measurement method for residual stress proximate aninterlayer in a multilayer thermal barrier coating system. This methodfocuses on detecting compressive stresses that accumulate at theboundary between the interlayer and the outermost thermal barriercoating by detecting the shift in frequency of light emitted byfluorescing chromium ions in the alumina interlayer. However,significant stresses at that boundary may not appear until after theoutermost barrier layer has seriously deteriorated. Furthermore, thestresses at the boundary are not useful indicators of the past-serviceconditions of the component itself. Therefore, such a method is not veryuseful in timely forewarning a need for repairing or replacing theengine component.

[0010] Therefore, there is a continued need to provide a simplenon-destructive method for determining the past-service condition of athermal barrier coating of a component used at high temperature in aturbine engine. It is also very desirable to provide a method by whichthe remaining useful life of the underlying component may be determinedor estimated. Furthermore, it is also very desirable to provide such amethod so that maintenance of turbine engine components may be performedonly on an as-needed basis rather than on a fixed schedule.

SUMMARY OF THE INVENTION

[0011] The present invention provides a method for determining at leastone of past-service conditions and remaining useful life of at least oneof a component of a combustion engine and a thermal barrier coatingthereof, which component is used in the hot-gas path of the combustionengine. The method of the present invention comprises (1) providing acombustion-engine component comprising a thermal barrier coating thatcomprises at least one photoluminescent (“PL”) material that can beexcited by radiation at a first wavelength range and emits radiation ata second wavelength range different from the first wavelength range inresponse to the exciting radiation; the radiation emitted at the secondwavelength range having a characteristic property that correlates withan amount of a crystalline phase in the thermal barrier coating, whichamount increases as the combustion-engine component is exposed toelevated temperatures; (2) directing radiation having the firstwavelength range at the thermal barrier coating of the combustion-enginecomponent; (3) measuring the characteristic property of radiation havingthe second wavelength range; (4) determining the amount of thecrystalline phase present in the thermal barrier coating from thecharacteristic property of radiation having the second wavelength range;and (5) inferring at least one of past-service conditions and remaininguseful life of the thermal barrier coating from the amount of thecrystalline phase.

[0012] According to one aspect of the present invention, the thermalbarrier coating comprises yttria-stabilized zirconia.

[0013] According to another aspect of the present invention thecrystalline phase is the monoclinic phase of zirconia.

[0014] The present invention also provides an apparatus for determiningat least one of past-service conditions and remaining useful life of atleast one of a component of a combustion engine and a thermal barriercoating thereof, which component is used in the hot-gas path of thecombustion engine. The apparatus comprises (1) a source of radiationhaving a first wavelength range directed at the thermal barrier coatingthat comprises at least one PL material capable of emitting radiationhaving a second wavelength range in response to an excitation by theradiation having the first wavelength range; (2) a radiation detectorbeing capable of detecting the radiation having the second wavelengthrange and being disposed to receive and measure a characteristicproperty thereof; and (3) means for relating the characteristic propertyof radiation having said second wavelength range to one of an amount ofa crystalline phase, past-service conditions, and remaining useful lifeof the combustion-engine component.

[0015] Other features and advantages of the present invention will beapparent from a perusal of the following detailed description of theinvention and the accompanying drawings in which the same numerals referto like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is the phase diagram showing various phases of zirconia andzirconia stabilized with yttria.

[0017]FIG. 2 shows the emission spectra from the PL Y₂O₃:Eu³⁺ materialin samples of YSZ thermal barrier coatings that have been treated atdifferent temperature to produce only the tetragonal phase.

[0018]FIG. 3 shows the emission spectra from a sample containing onlythe tetragonal phase and one containing 36% tetragonal phase and 64%cubic phase.

[0019]FIG. 4 shows a comparison of the emission spectra of a samplecontaining 65% cubic phase and 35% monoclinic phase with samplescontaining only tetragonal and cubic phase.

[0020]FIG. 5 shows a comparison of the emission spectra of two samplescontaining different levels of monoclinic phase and containing onlytetragonal and cubic phases.

[0021]FIG. 6 shows a correlation of the ratio of intensities of peaks at615 nm and at 605 nm to the amount of monoclinic phase in the thermalbarrier coating.

[0022]FIG. 7 is a schematic diagram of an apparatus of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] As used herein, the term “combustion engine” means any enginethat generates work using energy derived from combustion of a fuel. Acombustion engine can include components or assemblies of componentsthat convert the energy of the combustion to other forms of energy.Thus, the term “combustion engine” includes turbine engines.

[0024] As used herein, the term “elevated temperatures” meanstemperatures greater than about 700° C.

[0025] Yttria-stabilized zirconia is a material commonly used as thermalbarrier coating (“TBC”) for components used in the hot-gas path ofgas-turbine engines. This material has many advantages such as hightolerances for thermal shock, low thermal conductivity, and a highermelting point than most oxides. However, one of the limitations of purezirconia is that it undergoes phase transitions as temperature changes.At temperatures less than 1170° C., the equilibrium phase of zirconia ismonoclinic. Between 1170° C. and 2370° C. it is tetragonal, and above2370° C. it is cubic. The tetragonal-to-monoclinic phase transitionoccurring as temperature drops to 1170° C. is accompanied by anapproximate 3 percent change in volume that causes stresses in thecoating and can lead to spallation. FIG. 1 shows that addition of yttriacan stabilize zirconia in its cubic or tetragonal phase at temperaturesmuch lower than 1170° C. However, full stabilization to the cubic phasecompromises the cyclic thermal fatigue life. Therefore, zirconia istypically partially stabilized with 6-8 percent by weight of yttria(“YPSZ”). The term “yttria-stabilized zirconia” also includes YPSZ. WhenYPSZ is plasma sprayed in the process of forming the TBC, the moltenparticles are quenched to form a metastable tetragonal phase having thesame composition as the molten YPSZ. This metastable tetragonal phasedoes not transform immediately into the monoclinic phase when the TBC iscooled rapidly to room temperature. However, during typical use of theengine component that includes extended time at elevated temperatures,the metastable tetragonal phase can undergo a transformation into anequilibrium mixture of the cubic phase and a tetragonal phase that cantransform into the monoclinic phase. Since the monoclinic phase haslattice parameters different from those of the cubic and tetragonalphases, the growth of this phase in the TBC would eventually lead tospallation as discussed above. Such a spallation, in turn, would lead toexposure of the underlying alloy material of the engine component toextreme temperatures that would accelerate its failure. As the enginecomponent is exposed to elevated temperatures, it is therefore desirableto determine or monitor the progressive growth of the monoclinic phasein the TBC.

[0026] The applicants have discovered that the growth of the monoclinicphase in the TBC can be detected quantitatively and non-destructively bymeasuring the intensity of a characteristic peak in the emissionspectrum of a photoluminescent material incorporated into the TBC. Sucha characteristic peak is unique to the presence of the monoclinic phase,and thus can provide an unambiguous determination of past-servicethermal conditions of the engine component and/or its remaining usefullife before it must be replaced or a new TBC must be applied. Thepresent invention provides a novel method and apparatus for such adetermination. Since emission spectrum is measured on the componentitself, the data is directly attributed to the condition of thecomponent being tested and the condition of an untested component neednot be ambiguously inferred from another destructively tested.

[0027] The method of the present invention comprises (1) providing acombustion-engine component comprising a thermal barrier coating thatcomprises at least one photoluminescent (“PL”) material that can beexcited by exciting radiation at a first wavelength range and emitsradiation at a second wavelength range different from the firstwavelength range in response to the exciting radiation; the radiationemitted at the second wavelength range having a characteristic propertythat correlates with an amount of a crystalline phase in the thermalbarrier coating, which amount increases as the combustion-enginecomponent is exposed to elevated temperatures; (2) directing radiationhaving the first wavelength range at thermal barrier coating of thecombustion-engine component; (3) measuring the characteristic property,such as an intensity of a characteristic peak, of radiation having thesecond wavelength range; (4) determining the amount of the crystallinephase present in the thermal barrier coating from the characteristicproperty of radiation having the second wavelength range; and (5)inferring at least one of past-service conditions and remaining usefullife of the thermal barrier coating from the amount of the crystallinephase.

[0028] The present invention also provides a method and apparatus fordetermining past-service conditions and/or remaining useful life of thethermal barrier coating and the underlying component. Such past-serviceconditions and/or remaining useful life can be determined reasonablyaccurately by the amount of the monoclinic, phase present in the TBCbecause such an amount can be correlated to the thermal service historyof the component and the expected useful life of a new componentsimilarly constructed. Once the condition and/or the remaining life ofthe TBC has been determined, the remaining useful life of the enginecomponent also can be determined.

[0029] In another embodiment of the present invention, more than onetype of activator ions may be incorporated in the photoluminescent hostmaterial in the TBC, each type of activator ions being incorporated in afraction of the thickness of the TBC and each being capable of giving adistinct emission characteristic. As the engine component is exposed toelevated temperature during use, a first type of activator ions can emita spectrum with a first characteristic that varies with the service timeat the elevated temperatures. If the TBC is eroded, for example, becauseof spallation of an outer portion of the TBC, a second type of activatorions incorporated in a deeper layer of the TBC becomes exposed toexcitation radiation and emits a spectrum with a second characteristic.Such a manifestation of difference in spectrum characteristics canprovide a measure of an erosion of the TBC and, thus, an estimate of theprotection that the remaining potion of the TBC can afford and of theremaining useful life of the TBC and the engine component. In addition,the spectrum having a second characteristic of the second type ofactivator ions can provide the historical service condition of thatportion of the TBC wherein the activator ions of the second type reside.For example, as the engine component is put in service, there is agradient in the average temperature across the thickness of the TBC.This gradient in average temperature can result in a gradient in theamount of the monoclinic phase across the thickness of the TBC.Therefore, a new peak in the emission spectrum of the second type ofactivator ions can provide information on this amount of the monoclinicphase in the intermediate portion of the TBC that is exposed after anerosion of the outer portion. The historical service condition, such asthe average temperature experienced by, the intermediate portion of theTBC can also be determined from such an emission spectrum of the secondtype of activator ions even in the case in which an erosion of the outerportion of the TBC has not completely or substantially occurred if lightpipes have been formed or constructed into the TBC and penetrate thesame. In such a case, exciting radiation can be provided to and emittedradiation can be obtained from a deeper portion of the TBC.

[0030] A method of the present invention may be implemented on line oroff line. In an on-line method, equipment is provided with the engine tocarry out the steps of the method. Measurements may be made continuouslyor at certain desired intervals. In an off-line method, measurements maybe made with the cooled-down engine component in place or removed fromthe engine.

[0031] The TBC of an engine component used in the method of the presentinvention comprises zirconia partially stabilized with yttria in a rangefrom about 6 to about 8 percent by weight. In one aspect of the presentinvention, the yttria is doped with one of the rare earth-metal ions,such as Eu³⁺ present in europia (Eu₂O₃) to render it stronglyluminescent in the visible wavelength range in response to an excitationby ultraviolet (“UV”) radiation having a wavelength of about 253 nm. Asused herein, UV radiation includes radiation having wavelengths fromabout 100 nm to about 400 nm. Other dopants; for examples, otherrare-earth metals, that respond to excitation energy other than UV maybe used with yttria. For example, samarium doped in yttria may beexcited at about 400 nm to emit in the visible range. Still other oxidesmay be advantageously used in place of yttria depending on thecircumstances. In such cases, dopants may be chosen to provide emissionin a desired wavelength range; for example, in a range that is mostsuitable for the chosen radiation detector. Another exemplary dopant isterbium which responds to exciting radiation having wavelength betweenabout 280 nm and about 310 nm and emits with a strong peak at about 543nm. Still another exemplary dopant is dysprosium which responds toexciting radiation having wavelength about 350 nm and emits with astrong peak at about 572 nm. Erbium is another suitable dopant thatresponds to exciting radiation having wavelength of about 380 nm andemits with a strong peak at 563 nm. Praseodymium is another suitabledopant that responds to UV exciting radiation having wavelength about283 nm and emits a strong peak at wavelength about 630 nm. Otherrare-earth metal dopants that also may be used are gadolinium, holmium,and thullium.

[0032] Typically, an activator ion excited by radiation in onewavelength range emits in another longer wavelength range. For example,when excited in the UV range, typical activator ions emit strongly inthe visible-light wavelengths. When excited in the visible-lightwavelength range, they typically emit in the longer-wavelength visiblerange or in the near infrared (“near IR”) range.

[0033] The present invention also encompasses TBCs that comprisezirconia stabilized with one or more metal oxides other than yttria,such as yttrium aluminum oxide garnet, calcia, magnesia, india, scandia,and/or ytterbia.

[0034] Specimens were fabricated, each having a TBC made from oxidepowders with nominal composition of 5 weight percent (“wt %”) E₂O₃/7.6wt % Y₂O₃/87.4 wt % ZrO₂ to demonstrate the method and apparatus of thepresent invention. The specimens were heat-treated in air at a range oftemperatures between about 900° C. and about 2000° C. for differenttimes between about 40 minutes and about 1000 hours to change the phasecontent of the TBC. The fractions of the tetragonal, cubic, andmonolithic phases were measured using X-ray diffraction. The coatingshad 100% tetragonal phase in the as-fabricated condition before any heattreatment. At higher temperatures and longer times, some of thetetragonal phase is converted to the cubic phase. At the highesttemperature and longest time, the monoclinic phase was also observed.The fluorescent spectrum of Y₂O₃:Eu³⁺ was measured for each of theheat-treated specimens. As will be discussed more fully below, new peaksat about 615 nm and about 626 nm were observed in the emission spectrumwhen the monoclinic phase was present. Thus, the intensity of thesepeaks can be used to determine the presence and the amount of themonoclinic phase in the novel method of the present invention.

[0035]FIG. 2 shows emission spectra of six specimens (S1, S2, S3, S8,S9, and S14), the TBC of each of which has only the tetragonal phase.Each of the characteristic peaks of each spectrum substantially remainsat the same wavelength when only the tetragonal phase is present.

[0036]FIG. 3 shows the spectra of specimen S1 having 100% tetragonalphase and of another specimen S18 having 36% tetragonal phase and 64%cubic phase. Although the peaks of the spectrum of the latter shift toslightly shorter wavelengths they both consist of the samecharacteristic peaks, the intensity of each peak remaining substantiallyunchanged.

[0037]FIG. 4 shows the spectra of specimen S1 having 100% tetragonalphase, specimen S21 having 83% tetragonal phase and 17% cubic phase, andspecimen S20 having 65% cubic phase and 35% monoclinic phase. When themonoclinic phase is present a new strong peak at 615 nm and a new weakerpeak at about 626 nm appear. FIG. 5 compares the spectra of specimen S15having 38% tetragonal phase and 62% cubic phase, specimen S16 having 78%cubic phase and 22% monoclinic phase, and specimen S17 having 59% cubicphase and 41% monoclinic phase. As the amount of the monoclinic phaseincreases, the intensity of the peaks at 615 nm and 626 nm alsoincreases but the intensity of the peak at 605 nm decreases. Therefore,the ratio of the intensity of the peak at 615 nm to that of the peak at605 nm can be correlated to the amount of the monoclinic phase in theTBC, as is shown in FIG. 6, to provide a novel method for determiningthis quantity and, thus, for inferring the past-service thermalconditions and/or for estimating the remaining useful life of the enginecomponent. Therefore, the method of the present invention can provide aforewarning of the need to repair or replace the engine component beforea serious failure occurs.

[0038] The present invention also provides an apparatus for determiningat least one of past-service conditions and remaining useful life of athermal barrier coating of a component of a combustion engine, whichcomponent is used in the hot-gas path of the combustion engine. FIG. 7shows a schematic diagram of an apparatus 10 of the present invention.The apparatus 10 comprises (1) a source 20 of radiation 22 having afirst wavelength range directed at the thermal barrier coating 26 thatcomprises at least one PL material capable of emitting radiation 28having a second wavelength range in response to an excitation by theradiation 22 having the first wavelength range; (2) a radiation detector30 capable of detecting the radiation 28 having the second wavelengthrange and being disposed to receive and measure a characteristicproperty thereof; and (3) means 40 for relating the characteristicproperty of radiation 28 having said second wavelength range to one ofan amount of a crystalline phase, past-service conditions, and remaininguseful life of the combustion-engine component.

[0039] An apparatus 10 of the present invention can optionally compriselight pipes, such as optical fibers, used to transmit radiation from theradiation source to the TBC and/or to carry the emitted radiation fromthe TBC to the radiation detector. An apparatus 10 of the presentinvention can further comprise one or more systems of lenses 50 thatfocus the radiation emitted from the TBC into an optical fiber and/orone or more systems of optical filters that allow measurement ofintensity of light having certain specific wavelengths. In oneembodiment of the present invention, the radiation detector is aspectrophotometer.

[0040] A past-service condition that can be determined by a method or anapparatus of the present invention may be the extreme temperaturesbetween which or the average temperature at which the engine componenthas been exposed during service. It may also be the length of time forwhich the engine component has been in service above a certaintemperature.

[0041] The turbine engine components that may be inspected by a methodor an apparatus of the present invention may be turbine blades, turbinevanes, turbine shrouds, or combustion liners.

[0042] The radiation having the first wavelength range may be UVradiation in the range from about 100 nm to about 400 nm or visiblelight having wavelength in the range up to about 450 nm.

[0043] The means for relating the characteristic property of theradiation having the second wavelength range to one of an amount of acrystalline phase, past service conditions, and remaining useful life ofthe engine component can comprise a general-purpose minicomputer ormicrocomputer with associated peripheral input/output devices such asanalog-to-digital or digital-to-digital converters that can interfacebetween the radiation detector and the computer. Such a computer can beloaded with software that performs the data conversion, correlation,calculation, and outputting. The means for relating can also be analogor digital circuits built specifically for performing the functionsnoted above. The means for relating can also comprise one or morelogical circuits that perform some decision making functions that notifythe operator of the engine about the need for repair or replacement ofthe engine component.

[0044] The present invention also provides a TBC disposed directly on asurface of an engine component or on an intervening layer that is inturn disposed on a surface of an engine component. The TBC compriseszirconia and one or more metal oxides that are capable of retarding thepropensity of zirconia to form the monoclinic phase. Metal oxides suchas yttria, yttrium aluminum oxide garnet, calcia, magnesia, india,scandia, and ytterbia can provide such a capability. The metal oxidesare further doped with one or more ions that can fluoresce or luminescein response to exciting radiation to provide a characteristic propertyof the monoclinic phase. Rare-earth metal ions may be advantageouslyused for this purpose. Such a characteristic property may be, forexample, a new peak in the emission spectrum of the fluorescing orluminescing metal ions, a shift in the wavelength, or a change in theintensity of a certain peak in the spectrum.

[0045] In one aspect of the present invention, the TBC consistsessentially of zirconia and at least one metal oxide selected from thegroup consisting of yttrium aluminum oxide garnet, calcia, magnesia,india, scandia, and ytterbia. The metal oxide is doped with at least onerare-earth metal ion.

[0046] While various embodiments are described herein, it will beappreciated from the specification that various combinations ofelements, variations, equivalents, or improvements therein may be madeby those skilled in the art, and are still within the scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A method for determining at least one of past-service conditions and remaining useful life of at least one of a component of a combustion engine and a thermal barrier coating thereof, said method comprising: (1) providing a combustion-engine component comprising a thermal barrier coating that comprises at least one photoluminescent (“PL”) material that can be excited by a first radiation at a first wavelength range and emits a second radiation at a second wavelength range different from said first wavelength range in response to said first radiation; said second radiation having a characteristic property that correlates with an amount of a crystalline phase in said thermal barrier coating, which amount changes as said combustion-engine component is exposed to elevated temperatures; (2) directing said first radiation having said first wavelength range at said thermal barrier coating of said combustion-engine component; (3) measuring said characteristic property of said second radiation; (4) determining said amount of said crystalline phase present in said thermal barrier coating from said characteristic property of said second radiation; and (5) determining at least one of past-service conditions and remaining useful life of at least one of said component of said combustion engine and said thermal barrier coating thereof from said amount of said crystalline phase.
 2. The method according to claim 1, wherein said thermal barrier coating comprises zirconia stabilized with at least one material selected from the group consisting of yttria, yttrium aluminum oxide garnet, calcia, magnesia, india, scandia, and ytterbia.
 3. The method according to claim 1, wherein said thermal barrier coating comprises a material selected from the group consisting of yttria-stabilized zirconia and yttria-partially-stabilized zirconia.
 4. The method according to claim 3, wherein yttria is present at an amount from about 6 to about 8 weight percent of said thermal barrier coating.
 5. The method according to claim 1, wherein said PL material is yttria doped with at least one a rare-earth metal ion.
 6. The method according to claim 2, wherein said at least one material is doped with at least one rare-earth metal selected from the group consisting of europium, samarium, terbium, dysprosium, erbium, praseodymium, gadolinium, holmium, and thullium.
 7. The method according to claim 5, wherein said rare-earth metal is europium.
 8. The method according to claim 1, wherein said first wavelength range is an ultraviolet range.
 9. The method according to claim 1, wherein said first wavelength range is a visible light less than about 450 nm.
 10. The method according to claim 1, wherein said second radiation is a visible light.
 11. The method according to claim 1, wherein said second radiation is a near IR radiation.
 12. The method according to claim 1, wherein said crystalline phase is a monoclinic phase.
 13. The method according to claim 12, wherein said characteristic property is an intensity of a peak at about 615 nm in an emission spectrum.
 14. The method according to claim 12, wherein the step of determining said amount of said crystalline phase comprises correlating a ratio of intensities of peaks at about 615 nm and about 605 nm with known amount of the monoclinic phase.
 15. The method according to claim 1, wherein the step of determining at least one of past-service conditions and remaining useful life of at least one of said component of said combustion engine and said thermal barrier coating comprises correlating said amount of said crystalline phase with data selected from the group consisting of known historical temperature and time to failure of an engine component.
 16. A method for determining at least one of past-service conditions and remaining useful life of at least one of a component of a combustion engine and a thermal barrier coating thereof, said method comprising: (1) providing a combustion-engine component comprising a thermal barrier coating that comprises at least two PL materials that can be excited by a first radiation at a first wavelength range and emits at least a second radiation in a second wavelength range different from said first wavelength range in response to said first radiation, said second radiation emitted by each of said PL materials having a different characteristic property attributable to each of said PL materials, said characteristic property correlating with an amount of a crystalline phase in said thermal barrier coating, which amount increases as said combustion-engine component is exposed to elevated temperatures; (2) directing said first radiation having said first wavelength range at said thermal barrier coating of said combustion-engine component; (3) measuring said characteristic property of said second radiation; (4) determining said amount of said crystalline phase present in said thermal barrier coating from said characteristic property of said second radiation; (5) determining a remaining amount of said thermal barrier coating; and (6) determining at least one of past-service conditions and remaining useful life of at least one of said component of said combustion engine and said thermal barrier coating thereof from said amount of said crystalline phase and said remaining amount of said thermal barrier coating.
 17. An apparatus for determining at least one of past-service conditions and remaining useful life of at least one of a component of a combustion engine and a thermal barrier coating thereof, said apparatus comprising: (1) a source of first radiation having a first wavelength range, said first radiation being directed at said thermal barrier coating that comprises at least one PL material capable of emitting a second radiation having a second wavelength range in response to an excitation by said first radiation; (2) a radiation detector being capable of detecting said second radiation and being disposed to receive and measure a characteristic property thereof; and (3) means for relating said characteristic property of said second radiation to one of an amount of a crystalline phase in said thermal barrier coating, past-service conditions, and remaining useful life of said combustion-engine component.
 18. The apparatus according to claim 17, wherein said thermal barrier coating comprises zirconia stabilized with at least one material selected from the group consisting of yttria, yttrium aluminum oxide garnet, calcia, magnesia, india, scandia, and ytterbia.
 19. The apparatus according to claim 17, wherein said thermal barrier coating comprises a material selected from the group consisting of yttria-stabilized zirconia and yttria-partially-stabilized zirconia.
 20. The apparatus according to claim 19, wherein yttria is present at an amount from about 6 to about 8 weight percent of said thermal barrier coating.
 21. The apparatus according to claim 17, wherein said PL material is yttria doped with at least one a rare-earth metal ion.
 22. The apparatus according to claim 18, wherein said at least one material is doped with at least one rare-earth metal selected from the group consisting of europium, samarium, terbium, dysprosium, erbium, praseodymium, gadolinium, holmium, and thullium.
 23. The apparatus according to claim 21, wherein said rare-earth metal is europium.
 24. The apparatus according to claim 17, wherein said first wavelength range is an ultraviolet range.
 25. The apparatus according to claim 17, wherein said first wavelength range is a visible light less than about 450 nm.
 26. The apparatus according to claim 17, wherein said second radiation is a visible light.
 27. The apparatus according to claim 17, wherein said second radiation is a near IR radiation.
 28. The apparatus according to claim 17, wherein said crystalline phase is a monoclinic phase.
 29. The apparatus according to claim 28, wherein said characteristic property is the intensity of a peak at about 615 nm in an emission spectrum.
 30. The apparatus according to claim 17, wherein said radiation detector is a spectrophotometer.
 31. The apparatus according to claim 28, wherein said relating said characteristic property of said second radiation to said amount of said crystalline phase comprises correlating a ratio of intensities of peaks at about 615 nm and about 605 nm with known amount of the crystalline phase.
 32. The apparatus according to claim 28, wherein said relating said characteristic property of said second radiation to said past-service conditions comprises correlating a ratio of intensities of peaks at about 615 nm and about 605 nm with known temperatures to which said engine component has been exposed.
 33. The apparatus according to claim 28, wherein said relating said characteristic property of said second radiation to said remaining useful life of said engine component comprises correlating a ratio of intensities of peaks at about 615 nm and about 605 nm with a known time to failure of said engine component.
 34. A thermal barrier coating on an engine component, said thermal barrier coating comprising zirconia and at least one metal oxide that is capable of retarding a formation of a zirconia monoclinic phase when said engine component is exposed to elevated temperatures, said at least one metal oxide being doped with at least one metal ion that is excitable by a first radiation having a first wavelength range and emits a second radiation having a second wavelength range in response to an excitation by said first radiation, a spectrum of said second radiation having a characteristic property that is identifiable with and quantifiable in relation to an amount of the zirconia monoclinic phase.
 35. A thermal barrier coating on an engine component, said thermal barrier coating consisting essentially of zirconia and at least one metal oxide that is capable of retarding a formation of a zirconia monoclinic phase when said engine component is exposed to thermal cycling, said at least one metal oxide being doped with at least one metal ion that is excitable by a first radiation having a first wavelength range and emits a second radiation having a second wavelength range in response to an excitation by said first radiation, a spectrum of said second radiation having an characteristic property that is identifiable with and quantifiable in relation to an amount of the monoclinic phase; wherein said at least one metal oxide is selected from the group consisting of yttrium aluminum oxide garnet, calcia, magnesia, india, scandia, and ytterbia.
 36. The thermal barrier coating on an engine component according to claim 35, wherein said at least one metal ion is selected from the group consisting of rare-earth metal ions. 